1. Field of the Invention
This invention is related to a bonding method of electronic circuit devices.
2. Description of Related Art
When some of electronic devices in an apparatus become unnecessary due to a change in specification or the like and are required to be removed from their substrates, it is typical to conduct re-heating of solder joints bonding between terminal pads of these unnecessary electronic devices and bond pads of a device mounting pattern formed on the substrates.
However, at the same time, the re-heating of solder joints induces the formation of intermetallic compound layers of less wettability on the bond pads of the device mounting pattern on the substrate. This becomes a problem since these intermetallic compound layers prevent terminal pads of newly mounted electronic devices from creating stable bonds.
Japanese Patent Application Publication (KOKAI) No. 64-10634 suggested a semiconductor integrated circuit apparatus in which electronic devices may be removed from a substrate by using mechanical force without damaging the substrate to be recycled.
In the semiconductor integrated circuit apparatus of prior art, terminal pads of the electronic devices are soldered on the corresponding bond pads of the device mounting pattern formed on the substrate using a face down bonding method. Here, each of the bond pads of the device mounting pattern is provided so as to have a larger surface area compared to the corresponding terminal pad of the electronic device so as that the mechanical force (the force to break the solder joint) applied during the removal of the electronic device may be concentrated toward a side of the electronic device.
Accordingly, the substrate to be recycled is hardly damaged while the unnecessary electronic devices may be destroyed. This type of solder bonding has an advantage when frequent replacements of the electronic devices mounted on the substrate are anticipated.
There is another method of bonding between the bond pads of the device mounting pattern formed on the substrate and the terminal pads of the electronic devices. It is known as a flip-chip method in which solder bumps formed on the terminal pads of the electronic devices are melted to form bonding between the terminal pads of the electronic devices and the bond pads of the device mounting pattern formed on the substrate so as to establish electrical connections between them.
The flip-chip method has an advantage in that the electronic device may be soldered with an appropriate attitude at a predetermined location since a position of the electronic device with respect to the substrate is self-aligned due to a surface tension of the molten solder.
In order to fully utilize the self-alignment function in the prior art, it is necessary to remove an oxidized film on the surface of the solder bump formed on the terminal pad of the electronic device before the soldering, by applying fluxes on the solder bump.
Reliability of the electronic circuit apparatus which is the final product largely depends on the stability of bonds between the device mounting pattern and the terminal pads of the electronic device. The published specification of Japanese Utility Model Application No. 54-155903 teaches a semiconductor chip mount body in which optimization in the shape of solder joint, which bonds between the bond pads of the device mounting pattern formed on the substrate and the terminal pad of the electronic device, is attempted so as to provide long term stability of the bond.
As shown in FIG. 8A, insulation pads 814 are formed on a substrate 804 used in the semiconductor chip mount body at positions which do not correspond to any terminal pads 802 of an electronic device 801, in addition to bond pads 805 of a device mounting pattern. To mount the electronic device 801, solder pastes 815 are printed in a sheet-like form on the insulation pads 814. The solder paste 815 has a higher melting point than that of solder bumps 806, 807 formed on the terminal pads 802 of the electronic device 801 and the bond pads 805 of the device mounting pattern.
The flux is applied on each of the solder bumps 806 formed on the terminal pads 802 of the electronic device 801 and the solder bumps 807 formed on the bond pads 805 of the substrate 804 to remove the surface oxidized film.
After the electronic device 801 is disposed on the substrate 804 at a predetermined position, both of them are heated up to a predetermined temperature. The solder bumps 806, 807 formed on the terminal pads 802 and the bond pads 805 melt first. The molten solder wets and spreads over surfaces of the bond pad 805 formed on the substrate 804 and the terminal pad 802 of the electronic device 801, as shown in FIG. 8B.
Then, as shown in FIG. 8C, the solder sheets 815 begin to melt. Here, the molten solder sheets 815 do not wet or spread over the surface of the electronic device 801. The molten solder sheets 815 hold up the electronic device 801 by their surface tensions, thereby causing the solder joints 809 to form a drum type shape or a pillar type shape, both of which are stable against external stresses, between the bond pads 805 formed on the substrate 804 and the terminal pads 802 of the electronic device 801.